Work in prokaryotes and simple eukaryotes has shown that the synthesis of many aminoacyl-tRNA synthetases is accelaterated when these organisms are grown under conditions in which the supply of the cognate amino acid is restricted. Preliminary results in our laboratory suggest the the synthesis of the isoleucyl-tRNA synthetase of chinese hamster ovary cells is accelerated during periods of isoleucine deprivation. When isoleucine is added back to isoleucine- deficient cultures a significant burst of isoleucyl-tRNA synthetase activity is observed. This burst is interpreted as resulting from translation of accumulated mRNA for isoleucyl-tRNA synthetase. Studies will be carried out to distinguish between a decreased pool of free isoleucine or of isoleucyl-tRNA IIe as the signal for increased synthesis. We also propose to use this system to distinguish between factors affecting mRNA synthesis and degradation and those affecting enzyme synthesis and degradation. Experiments will also be performed to see if other aminoacyl-tRNA synthetases of animals cells are synthesized at accelerated rates in response to a deficiency of the cognate amino acid.